Maryland Geological Survey 453 



Subgenus PLETHOMYTILUS Hall 

 MyTILARCA (PLETirOMYTILUS) ROWEI n. Sp. 



Plate LXXV, Fig. 7 



Description. — Shell equivalve; not strongly inequilateral; gibbous; 

 broadly ovate ; umbonal slope expanding somewhat rapidly from umbones 

 to about one-fourth or cue-third the length of the shell, then arching 

 broadly and gently to the postero-basal margin, sloping gradually to the 

 postero-dorsal and suddenly to the antero-basal margin; postero-dorsal 

 margin almost rectilinear to about three-fourths the length of the shell, 

 posterior extremity subcircularly rounded, basal margin broadly rounded, 

 anterior margin rectilinear (?) to beak; beaks prominent, approximate. 

 Surface of interior smooth, interrupted irregularly by broad, often dis- 

 continuous, concentric furrows. Hinge-line not seen. 



Greatest length 73 mm. ; greatest width 55 mm. 



All the specimens seen are casts of the interior, but there is little 

 doubt as to the generic affinities. The naming of the species is a humble 

 compliment to the late Dr. K. B. Eowo. 



Occurrence. — Oriskany Formation, Eidgely Member. Hancock. 



Collection. — Maryland Geological Survey. 



[Ohern.] 

 Genus AMPHICCEUA Hall 

 AMPHICQiLIA ULRICHI n. Sp. 



Plate LXXV, Figs. S-10 



Description. — Broadly triangular subovate in outline. Length and 

 height about equal. Valves equal, somewhat gibbous from the center to 

 the beaks, sloping abruptly to the anterior margin, while the decline from 

 the most gibbous portion of the shell to the posterior and ventral margins 

 is more gradual, giving to the shell a cuneiform profile. The cardinal line 

 arcuate. Cardinal slopes concave in the vicinity of the beak in the casts. 

 Anterior and posterior cardinal margins and ventral margin regularly 

 rounded. In the posterior portion of the shell at the point of greatest 

 length, the shell is extended making the form inequilateral. The umbones 



